The present invention relates to ion sources having at least two ionization chambers, in particular for forming chemically reactive beams of ions; it relates more particularly to ion sources of this type having a long life span.
Ion sources with two ionization chambers of the "duoplasmatron" and "duopigatron" type and ion sources having three ionization chambers of the "triplasmatron" type are, in particular known.
One ion source of the duoplasmatron type comprises a succession of a hot cathode, an intermediate electrode and an anode pierced with a discharge hole through which exits a plasma jet formed by the electrons and by the positive ions produced by this source. From the plasma jet the ion beam is formed by the action of a magnetic field. In the duoplasmatron, an arc is produced between the cathode and the anode, this arc being constricted in the vicinity of the discharge hole of the anode under the effect of an electrostatic action caused by the intermediate electrode and a magnetic lens action created between the anode and the intermediate electrode. The first ionization chamber between the cathode and the intermediate electrode is followed by the second chamber between the intermediate electrode and the anode.
The duopigatron is distinguished from the duoplasmatron by the fact that in the duopigatron a fourth electrode, disposed downstream of the anode, is brought, by means of an auxiliary voltage source, to a negative potential with respect to that of the anode thus forming an anti-cathode which is pierced with a discharge hole. The fourth electrode plays the role of second pole of the magnetic lens in place of the anode of the duoplasmatron.
Finally, the triplasmatron, which forms the subject matter of a French patent No. 2 156 978 filed on the Oct. 13, 1971 in the name of l'Agence Nationale de Valorisation de la Recherche, is formed by a duoplasmatron with, downstream of the discharge hole of the anode, a fourth electrode, called an expansion dish, which is maintained at a positive potential with respect to the anode. This fourth electrode preferably has the form of an enclosure, with an inlet aperture, forming the third ionization chamber which receives a jet of electrons and positive ions coming from the duoplasmatron and an outlet opening for forming an electron beam of positive ions and/or negative ions.
In these three known types of ion sources, the cathode is brought to a negative potential with respect to the intermediate anode by means of a DC generator. The DC potential difference thus created between the cathode and the intermediate electrode creates a field which serves for producing a plasma in the first chamber between these two electrodes.
Though such ion sources are used on the relatively large scale for different applications at the research stage, e.g., for implantation of ions, production of chemically reactive ion beams (formation of oxygenated, fluorinated and chlorinated compounds for example), technology of integrated circuits (oxidation of the semi-conductors, etching of the integrated circuits by means of a reactive ion beam, and diagnosis of integrated circuits), they have however the disadvantage of a reduced life span incompatible with industrial development, particularly in the case of operation with gases reacting chemically with the hot thermoemissive cathode. This is because the partial reactive gas pressure in the vicinity of the hot cathode is high, even for the case of the triplasmatron which already represents an improvement in that the reactive gas is only introduced therein in the third chamber and under a pressure less by a factor of 20 to 100 than that existing in the first chamber whose typical value is 10.sup.-1 torr.
It was thought that the life span of ion sources having two or three chambers of the above mentioned type could be increased by using not a hot cathode, but a cold cathode for reducing the chemical reactions between some gases and the cathode, but the performances of the ion source are reduced, insofar as the ionization efficiency, the energy dispersion of the emitted ions, the emittance, the brightness and the reproducibility of the performances are concerned, when a cold cathode is used.